OTP application entry for Rheo.
By default Rheo does not auto-start an instance. Host applications supervise
{Rheo, opts} themselves.
Set config :rheo, start_on_application: true together with
config :rheo, backend: Rheo.Backend.ETS (or {module, opts}) to start the
default instance under this application. The legacy :mongo_url key is
honoured only when :backend is unset and Rheo.Backend.Mongo is available.